The regulation of sodium excretion by the kidneys occurs through a variety of neural, humoral, and intrarenal regulatory mechanisms. Acute unilateral nephrectomy (AUN) increases electrolyte excretion by the remaining kidney via a mechanism which can be described in terms of a circulatory reflex arc. The afferent limb of this reflex arc involves an acute rise in blood pressure which occurs after AUN and which is sensed by the carotid sinus baroreceptors. The efferent limb involves pathways which are not fully defined, but which do not appear wholly attributable to activity of the renal nerves or to prostaglandins. Current studies will explore these relationships further by examining factors such as volume expansion which may amplify the response to AUN.